At present there are monitoring systems of the chemical reagent or the colored ink type on the market which are not always satisfactory, because these systems do not give any indication of the duration of exposition (for example a brutal change of color of a reagent at a given temperature, whatever the duration), or in case they give an estimate of the duration there is no infornation about the variations of temperature to which the object under supervision has been submitted, (for example, a system of strips which color themselves progressively depending on the time since a certain temperature is reached; the speed of absorption is the same whichever the temperature to which the object has been submitted).